An accessory drive generally comprises a drive pulley connected to a crankshaft of an internal combustion engine of the motor vehicle, at least a second and a third pulley respectively connected to an alternator and to an accessory, for example a hydraulic pump, and a belt for connecting the pulleys together.
During the cranking step, until combustion reaches a steady condition and the crankshaft is driven by a starter motor, combustion irregularities which induce a fluctuating torque on the drive occur.
Such fluctuating torque interacts with the inertia of the accessories rotationally driven by the drive, and in particular with that of the alternator which is the accessory with the highest inertia. The consequent stresses may prejudice the belt duration; this problem is particularly felt in motor vehicles of recent production, in which the electrical absorption is always higher and consequently larger, more powerful alternators are needed.
Hub-pulley assemblies having a rotationally disengageable pulley are known comprising a hub rigidly connected to the alternator shaft, a pulley fed by the accessory drive belt and a torsional disengaging device arranged between the pulley and the hub, to disengage the alternator when the speed of the alternator shaft exceeds the speed of the pulley.
In particular, the known torsional disengaging devices comprise for example a helical spring having an end portion rigidly connected to the pulley and an engaging portion having a plurality of turns wound about a cylindrical surface integral with the hub.
The spring is wound in a direction so that, when the pulley tends to exceed the angular speed of the hub, the turns are tightened about the cylindrical surface and generate a friction torque which rigidly engages the pulley and the hub.
On the contrary, when the angular speed of the alternator shaft tends to exceed that of the pulley, the turns of the spring tend to disengage from the cylindrical surface and allow the relative rotation of the hub with respect to the pulley, to prevent the inertia torque of the alternator from stressing the belt.
Recently, accessory drives have been made comprising a reversible electrical machine working both as a motor during cranking and as an alternator driven by the internal combustion engine.
If the known pulley assemblies were fitted or the shaft of the reversible electrical machine, they would slip during cranking because in such condition the shaft of the reversible electrical machine drives the drive and its rotational speed necessarily tends to exceed that of the pulley. Therefore, the known pulley assemblies cannot be used in combination with a reversible electrical machine.